It was added to the 3.6 branch on May 14.You can try it now if you have that version.

Only the SpineAnimationState and SpineSkeletonFlip clip types are included for now since those are the types that act most predictably. (It just makes AnimationState calls to a target SkeletonAnimation's AnimationState, or flips the Skeleton)We'll probably add a separate clip types soon for SkeletonGraphic since their types aren't reconcilable outside of sharing an interface, which currently isn't a supported property type by Unity editor.

Editor preview may not look exactly the same as at runtime, and backwards playback is not supported. We're hoping to improve things as we go though a lot of it depends on the type of exposed API Unity's Timeline and Playables feature has.